In today's posting of the Momentus XT review, Anand (nearly) ends it with this:
"Longer term it's unclear to me whether hybrid drives like the Momentus XT will fill the gap left by SSDs or if software based caching technologies combined with NAND on motherboards will be the preferred route."
Oddly, I wondered much the same a few years back when Sun (pre-Oracle) announced its flash "appliance". That, of course, was for near-mainframe level servers, but was an early assault on the flash-as-HDD approach to using flash. There've been discussions about, particularly in the context of PCIe (Fusion-io, of course) "drives", whether there need be this SSD-as-HDD approach. More folks are talking about storage now as just flash, pretty much, directly wired to the cpu and/or memory manager. As Linus said all those years ago, it will change how file systems are built; if we bother with them at all.
Not to mention, this tickled a lower brain stem memory: winFS. Go spend some time with the WikiPedia article. Mayhaps SSD will be the crutch needed?